They appear also, but not so frequently, in north-eastern Bulgaria, including the Dobrudza. This means that these two regions constituted the main gates of the Slavonic infiltration into the Peninsula. This is quite natural, because the two regions concerned were difficult to guard even for a well organised Byzantine army; the former is very mountainous and the latter was densely wooded in antiquity.

These results are confirmed by the study of hydronyms, which are very resistant to change, especially names of large rivers; they often survive repeated changes of population. Statistics of the names of the larger rivers in Bulgaria show the following pattern [6]: Origin Large rivers Medium-sized rivers Thracian (pre-Slavonic) 70 per cent 15 per cent Bulgarian 7 per cent 56 per cent

The geographical distribution of the Slavonic hydronyms in Bulgaria (see Fig. 3) is as follows. Slavonic names of rivers are frequent in western and north-western Bulgaria, but they are almost missing in eastern and especially in south-eastern Bulgaria. This means that the central zone of the Peninsula was first Slavonicised, while the east followed much later. The statistical patterns of the hydronyms agree, therefore, with the conclusion drawn from the Slavonic toponyms of Procopius.

http://www.allempires.net/historical-maps-of-the-vlachs-regions_topic14351_page4.html .. an article of 1882, about the neo-latin population in the Balkans south of the Danube, that states that "..according to a Report of the Austrian Consulate in Bucarest the neo-latin speaking people in the Balkans are a total of 3.134.450, and are so divided: 400.000 in Serbia, 420.000 in Bulgaria, 289.750 in Bosnia-Herzegovina, 77.300 in Novi-Pazar, 220.000 in Eastern Rumelia, 1.450.000 in Western Rumelia, Macedonia and Albania, 137.000 in the territories that Greece obtained from the Treaty of Berlin and 140.000 in the rest of Greece…"